fix: AI Dockerfile — graceful underthesea fallback, don't hard-fail

Try underthesea 6.8.0, fallback to latest, warn if both fail.
NLP features degrade gracefully without underthesea.

Co-Authored-By: Claude Opus 4 (1M context) <noreply@anthropic.com>

libs/ai-services/Dockerfile

@@ -8,7 +8,8 @@ RUN apt-get update && \
     rm -rf /var/lib/apt/lists/*
 
 COPY pyproject.toml .
-# Install deps in order: core deps first, then underthesea (may need build tools)
+# Install core deps first, then underthesea separately (may need build tools)
+# underthesea 6.8.0 needs maturin/Rust — try pre-built wheel, fallback to 6.8.4 or skip NLP
 RUN pip install --no-cache-dir \
     "fastapi==0.115.0" \
     "uvicorn[standard]==0.32.0" \
@@ -18,8 +19,9 @@ RUN pip install --no-cache-dir \
     "pydantic-settings==2.5.0" \
     "httpx==0.27.0" \
     "slowapi==0.1.9" && \
-    pip install --no-cache-dir "underthesea==6.8.0" || \
-    pip install --no-cache-dir "underthesea==6.3.4"
+    (pip install --no-cache-dir "underthesea==6.8.0" 2>/dev/null || \
+     pip install --no-cache-dir "underthesea" 2>/dev/null || \
+     echo "WARNING: underthesea install failed, NLP features disabled")
 
 COPY app/ ./app/